1. Technical Field
The present invention relates to the configuration of pins used to join graphite electrodes into an electrode column. More particularly, the invention concerns a unique design for a pin, and the electrodes for which the pin is used.
2. Background Art
Graphite electrodes are used in the steel industry to melt the metals and other ingredients used to form steel in electrothermal furnaces. The heat needed to melt metals is generated by passing current through one or a plurality of electrodes, usually three, and forming an arc between the electrodes and the metal. Electrical currents in excess of 100,000 amperes are often used. The resulting high temperature melts the metals and other ingredients. Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace.
Conventionally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections or tangs, with at least one end of the electrodes comprising female threaded sections capable of mating with the male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes, and the pin therebetween, are referred to in the art as a joint.
Given the extreme thermal stress that the electrode and the joint (and indeed the electrode column as a whole) undergoes, mechanical/thermal factors such as strength, thermal expansion, and crack resistance must be carefully balanced to avoid damage or destruction of the electrode column or individual electrodes. For instance, longitudinal (i.e., along the length of the electrode/electrode column) thermal expansion of the electrodes, especially at a rate different than that of the pin, can force the joint apart, reducing effectiveness of the electrode column in conducting the electrical current. A certain amount of transverse (i.e., across the diameter of the electrode/electrode column) thermal expansion of the pin in excess of that of the electrode may be desirable to form a firm connection between pin and electrode; however, if the transverse thermal expansion of the pin greatly exceeds that of the electrode, damage to the electrode or separation of the joint may result. Again, this can result in reduced effectiveness of the electrode column, or even destruction of the column if the damage is so severe that the electrode column fails at the joint section. Thus, control of the thermal expansion of an electrode, in both the longitudinal and transverse directions, is of paramount importance.
Of course, the optimal way of achieving thermal compatibility between pin and electrodes is to form the pin of the same material from which the electrodes are formed; however, in conventional pin joints, the pin must be formed of a graphite material that is stronger than the material from which the electrodes are formed needs to be. If the pin is not formed of a stronger material, it would fail (i.e., suffer cracks and breakage) to an unacceptable degree while in use in an electrode column. In order to avoid this, the pin can be formed, e.g., of a graphite material of a higher density than that needed for the electrodes.
What is desired, therefore, is a pin/electrode joint having sufficient strength and integrity to reduce column damage in use and create a more stable joint, without a significant reduction in electrode performance. It is also highly desirable to achieve these property benefits without using high quantities of expensive materials and, most advantageously, by use of the same graphite materials to form the pin as are used to form the electrodes in order to more closely match the thermal properties of the pin and electrodes.